1. Field of the Invention
The present invention relates generally to implantable medical devices and, more particularly, to a temperature regulated implantable medical device.
2. Related Art
The use of implantable medical devices to provide therapy to individuals for various medical conditions has become more widespread as the advantages and benefits such devices provide become more widely appreciated and accepted throughout the population. In particular, devices such as hearing aids, implantable pacemakers, defibrillators, functional electrical stimulation devices such as Cochlear™ prostheses, organ assist or replacement devices, and other partially- or completely-implanted medical devices, have been successful in performing life saving and/or lifestyle enhancement functions for a number of years.
Many such implantable medical devices often include one or more sensors, processors, controllers or other functional mechanical, electrical or electronic components that are permanently or temporarily implanted in a patient. Many of these devices receive power and/or data from external components that are part of, or operate in conjunction with, the implanted medical device. In particular, many such implantable devices include a power source integrated into the device. Some larger systems include more than one implantable device of which one is a power source which provides power to another implantable device. Such power sources are typically rechargeable batteries although other types of power sources have be implemented as well.
One such type of medical device is a Cochlear™ implant system. Cochlear™ implant systems provide the benefit of hearing to individuals suffering from severe to profound hearing loss. Hearing loss in such individuals is due to the absence or destruction of the hair cells in the cochlea which transduce acoustic signals into nerve impulses. Cochlear™ implants essentially simulate the cochlear hair cells by directly delivering electrical stimulation to the auditory nerve fibers. This causes the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve.
Conventional Cochlear™ implant systems have generally included an external assembly directly or indirectly attached to the body of the patient (sometimes referred to herein as the recipient), and an internal assembly which is implanted in the patient. More recently, Cochlear™ implant systems have been designed such that all of the systems' components are implanted subcutaneously; that is there is no external assembly. Because such Cochlear™ implant systems are entirely implantable, they are commonly referred to as a “totally” implantable Cochlear™ implant. This results in a more versatile system providing the recipient with greater freedom and ability to use the implant in what would previously have been regarded as adverse environments, e.g. wet environments. Such a Cochlear™ implant system is described in greater detail in U.S. Patent Application Publication No. 2003/0171787, which is hereby incorporated by reference herein. However, unlike earlier Cochlear™ implant systems, the totally implantable system is powered by a rechargeable battery that receives power transcutaneously via an implanted receiver coil.
In a totally implantable Cochlear™ implant device as well as other implantable medical devices which include a power source, there is a risk that a malfunction in the components that utilize or regulate the power consumption of the implantable device, or a malfunction in the power source itself, may cause excessive heat to be released. For example, if a malfunction causes an implantable battery to short circuit, the battery may rapidly discharge causing a rapid rise in temperature. Such an increase in temperature can damage the battery and the implanted components. This may cause the medical device to cease operating which, if the device is life sustaining, can be catastrophic. Alternatively, component failure due to overheating may necessitate replacement of the implantable component, which requires surgery. Furthermore, should the heat be conducted to the housing of the implanted device, such heat can be conducted to the surrounding tissue causing discomfort and possibly necrosis.